There are more ATM Machines down toward Terminals 2 & 3 (Domestic) and more Currency Exchanges near International Arrivals Terminal #1 (Most U.S. arrivals). Currency Exchanges often charge a Minimum Commission, so it could to cost you $6.00 (ESP 1.000)
to exchange USD $10 or $20. On my most recent 3-trips to Spain in year 2,000, ATM's did not charge a minimum commission. They also give a better exchange rate.
I have never tried to pay for a Metro ticket with a credit card.
[This message has been edited by Eddie (edited 02-27-2001).]